The present invention relates to an X-ray computed tomography (CT) system for medical or industrial use, or more particularly, to improvement of quality of a three-dimensional display image, a multi-planar reconstruction (MPR) display image or a maximum intensity projection (MIP) display image.
In X-ray CT systems, an apparatus for generating cone-beam X-rays and a two-dimensional X-ray area detector opposed to the apparatus in order to detect the X-rays and structured with detector elements arranged in the form of a matrix are rotated about a center of rotation located between them. Projection data items produced from X-rays transmitted by a subject lying between the apparatus and the detector are collected, and images are reconstructed based on the collected projection data items. The reconstructed tomographic images are displayed on an image display means (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2003-334188 (pp. 9-10, FIG. 17)
In the X-ray CT system, when a helical scan is performed, the centers of tomographic images are, as shown in FIG. 12, successively deviated from one another along with the movement of a table made in a z direction. Moreover, when a conventional (axial) scan or a cine scan is performed, the centers of tomographic images are, as shown in FIG. 13, deviated from one another on an xy plane, on which the tomographic images are defined, along with the movement of the table made in the z direction. This is attributable to a warp of a cradle, subject's body motions, or the like.
In the past, a slice thickness in the z direction has been large and the dimension of a pixel has not been isotropic. Therefore, when tomographic images are three-dimensionally displayed, the precisions in the positions in x and y directions of each of the tomographic images may not be very high. However, the slice thickness in the z direction has become smaller these days. The dimension of a pixel has become isotropic nowadays. Consequently, as far as a three-dimensional display image, an MPR display image, or an MIP display image is concerned, the deviations of the centers of images on the xy plane pose a problem in terms of accuracy and image quality in that the deviations lead to continuous or discontinuous deviations or steps in the y direction.
However, tomographic images can be aligned on the xy plane using a reference that permits the alignment of the tomographic images on the xy plane, that is, using the continuity of a subject as a reference.